rare

This is the extremely rare, pre-war "Valise, Device, Prosthetic, Lower Limb, for the use of" and was designed to safely store and carry artificial legs for Royal Marines(as only the Royal Navy was allowed to serve with a peg leg until Douglas Bader one-upped them by having two fake limbs.)
Actually, I have never seen one of these before but couldn't resist the urge to be silly.